The electrochemical cells of non-aqueous storage batteries typically include an anode including an alkali metal such as lithium. The anode metal may be present as a pure metal or alloy, or else may be releasably intercalated in a material such as carbon. The cell further includes a liquid electrolyte solution containing an electrolyte salt which is preferably a compound of the anode metal and which is dissolved in one or more organic solvents; and a cathode of an electrochemically active material, also referred to as a cathode-active material. The cathode-active material typically is a chalcogenide of a transition metal. During discharge, alkali metal ions from the anode pass through the liquid electrolyte solution to the cathode-active material of the cathode where the ions are taken up, with the release of electrical energy. During charging, the current flow of ions is reversed. Alkali metal ions pass from the electrochemically active or cathode-active material of the cathode through the electrolyte solution to the anode.
Cells incorporating lithium as the anode metal provide high energy density. That is, such cells can store substantial amounts of electrical energy for a given size. Manganese dioxide is a promising cathode-active material for such lithium-based cells. MnO.sub.2 provides a high electrochemical potential against lithium, and hence a high energy density. Moreover, it is low in cost and readily available. Therefore, considerable effort has been devoted to development of cells using lithium as the anode metal and MnO.sub.2 as the cathode-active material. In particular, considerable effort has been devoted to development of Li.sub.x /MnO.sub.2 cells which can be repeatedly charged and discharged, commonly referred to as "secondary" cells.
If the cell charging process is continued beyond the desired fully charged condition, then irreversible damage can occur. The voltage across the cell depends upon the existing state of charge of the cell. At any given state of charge the cell has a corresponding voltage or potential. Accordingly, a damaging overcharge can be prevented by terminating the charging cycle when the voltage across the cell reaches the charge potential corresponding to the desired fully charged state.
If cycling continues, irreversible damage may occur in the cell, ruining the cell. In some cases, a hazardous situation can occur as the cells is driven beyond safe limits of operation such as, for example, overcharging. These hazardous situations are thought to result from undesirable reactions which may occur when the cell is subjected to abuse such as overcharging or operation at abnormally high temperatures.
One such undesired reaction is the reaction of the anode material with the electrolyte solvent. The problem is most acute in secondary cells, and particularly in secondary cells having an alkali metal anode as alkali metals are generally quite reactive. As cells are repeatedly cycled, the surface area of the anode, particularly those anodes made of metallic lithium, increases with repeated plating of lithium from the electrolyte onto the anode during recharge. The electrolyte-lithium contact surface area likewise increases. This generally reduces the tolerance of the cells to thermal and electrical abuse. It has been shown that very high surface area lithium is generated in cycling duty cycles with a small discharge current. The increased surface area tends to promote reaction between the anode metal and the electrolyte solvent.
Furthermore, reactions between the electrolyte solvent and the anode are generally exothermic, providing heat which merely drives the reaction further. The heat and gasses generated by such reactions can raise the pressure within the cell to the point where the cell casing ruptures, as by the opening of an overpressure relief device incorporated in the casing wall. This is commonly referred to as "venting." Venting releases the electrolyte from the cell, effectively terminating the useful life of the cell. Moreover, the vented materials may contaminate the surrounding equipment and can pose a safety hazard under some conditions.
These problems may be particularly acute in batteries with lithium anodes because lithium has a melting point of only about 180.degree. C. It is therefore possible to generate temperatures inside the battery as a result of electrolyte reaction when the battery is subjected to abnormal operation in an environment at an elevated temperature or overcharging which can lead to melting of the anode. Melting of the lithium can result in internal short circuit, leading to sudden release of electrochemical energy as heat, and hence to violent venting. Cells which short circuit or which exhibit forced discharge, the latter of which can occur when a low capacity cell is discharged within a series of cells with normal capacity, may also exhibit similar problems.
Another problem facing electrochemical cell designers is that of trading off performance for safety. For example, certain formulations of electrolyte solutions may be "safe" when used in a Lix/MnO.sub.2 cell in that they do not cause violent venting. However, these same cells may exhibit Type 1 venting after a relatively low number of cycles. Type 1 venting refers to a moderate venting condition in which the opening of the cell safety vent occurs to relieve generated pressure in a controlled manner. While no flame or exothermic thermal runaway occurs in type 1 venting, the electrolyte may leak from the cell and may be toxic or may degrade the plastics used in battery pack housings. As such, these cells exhibit poor performance and their useful life may be extremely limited and they may pose a slight safety risk. Other types of venting include: Type 2 venting which involves the opening of the cell vent accompanied by a mild flame; Type 3 venting which involves the opening of the cell vent accompanied by vigorous flame of up to 30 cm in length; and Type X venting in which there is insufficient time for the cell vent to open and violent explosion results. These latter venting conditions pose a more considerable safety risk. This classification system is sometimes used in the industry as a way of describing the reaction of cells when abused.
The industry has long searched for solutions to these problems. See, for example, Japanese Patent Application [Kokai] No. JP64-14879 (1989) and German Patent No. DE 3,024,151 (relating to a different electrode systems.) However, these attempts have failed to be completely satisfactory.
Therefore, there remains a need for an electrolyte solvent which has dramatically reduced reaction propensities within lithium batteries but which nonetheless provides acceptable performance.